The present invention relates to a communication terminal and a program executed in the communication terminal, in particular, the present invention can be preferably used for a communication terminal that acquires time information for verifying validity of a certificate in cryptographic communication from outside and holds the time information inside the communication terminal and a security program for detecting an attack to the time information.
In a V2X (Vehicle to X) communication (communication between vehicles, between road and vehicle, and the like) system, the reliability of time information is important to confirm an effective period of a certificate. This is to prevent a so-called replay attack in which an attacker receives a message that is normally transferred in the V2X system and transmits the message without change. Therefore, reliability is required for time information itself held by an on-vehicle terminal itself of a vehicle participating in the V2X communication.
Japanese Unexamined Patent Application Publication No. 2008-228051 discloses a cryptographic communication system in a road-vehicle communication system using a common key cryptography method. Every time an on-vehicle terminal enters an area where communication with a roadside machine is possible, the on-vehicle terminal acquires and stores GPS (Global Positioning System) information and transmits the acquired GPS information to a server, so that the on-vehicle terminal shares the same data with the server.